12AP4 Substitute

As an alternative to rebuilding 12AP4s, Dennis Choinski located a number
of 9 and 12 inch 40 degree radar tubes, with soft glass, in England. He
purchased one and had Clinton Electronics re-phosphor it and rebuild it with a new
gun. The project was a success, and cost about $300.
Following up on Dennis's leads, we procured 15 of the 12 inch tubes. A
poll was taken of owners of sets with 12AP4s, and there was enough
interest in making "12AP4 substitutes" to order the radar tubes from
England. They were shipped to Clinton, and will be re-phosphored and rebuilt
with new guns. These substitute tubes will cost just under $500,
including the cost of shipping the radar tubes from the UK, and
will work nicely in sets using 12AP4s. Both the rebuilt 12AP4s and the
substitute 12AP4s will have thin aluminized phosphors, which will be
much brighter than the originals, and won't be subject to ion burns.

We also ordered two 21 cm
versions of the same radar tube for 9AP4 substitutes.

Clinton has finished
rebuilding the first of these tubes. We have put it in our RCA TRK-12.
It produces an excellent, bright, sharp picture; the best I've ever seen
on a pre-1945 set. The only electrical change required is to connect the
filament lines in the power supply chassis to 6.3 volts. Here are photos:

A 12AP4 (left) and the substitute. Notice that the bulb is somewhat
shorter, as is the neck

The original 12AP4 (the cardboard safety cover
is missing)

The lower portion of the original tube,
showing the yoke mounting

The yoke mounting brackets were inverted to
accommodate the shorter substitute tube

The substitute, with a recessed cavity anode
cap installed

The substitute with the cardboard safety cover
installed

The new anode connector cable connected to the
one in the TRK-12

The splice is pushed up into the cardboard
safety cover to hide it

Top view of the substitute CRT showing the
cardboard safety cover

The tube comes with a socket, which we have temporarily
connected to a 6 pin plug salvaged from a tube. Once testing is
complete, we will connect the pins of the substitute CRT
directly to the plug and glue it to the end of the CRT.

A picture of taken of the screen of the
substitute tube. The picture is much sharper than it appears
here. I will take better photos later.